RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v11]

[Thomas Stuefe]

On Wed, 11 Sep 2024 12:47:30 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix FullGCForwarding initialization
>
> src/hotspot/share/memory/classLoaderMetaspace.cpp line 115:
> 
>> 113:   if (wastage.is_nonempty()) {
>> 114:     non_class_space_arena()->deallocate(wastage);
>> 115:   }
> 
> This code reads a bit strangely. I understand *what* it tries to do. It tries to give back any wasted memory from either the class space arena *or* the non class space arena to the non class space arena's freelist. I assume that we do this since any wastage is presumably too small to be used by our new 22-bit class pointers. However, this context will be lost on future readers. It should have at least a comment in the `if (wastage.is_nonempty())` clause explaining what we expect should happen and why. For example:
> 
> ```c++
> // Any wasted memory is presumably too small for any class.
> // Therefore, give it back to the non-class space arena's free list.

Yes. Some background:

- wastage can only occur for larger Klass* alignments (aka class space arena alignment property), so only for +COH (note to self, maybe assert)
- wastage is, by definition, not aligned to the required Klass* alignment, so it cannot be reused. Yes, its probably also too small

Yes, I will write a better comment.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1755111131